Extreme ultra-violet (EUV) light can be used in inspection tools which typically have a volume allocated to EUV creation, called the Source Area, and a volume allocated to optical inspection, called the Projection Optics Area. All apparatuses that create EUV light also create unwanted contamination or oxidation as part of that EUV generation. This contamination or oxidation can take the form of particles, ions, or gaseous chemicals. Retical inspection tools using EUV light can have a limited operational lifetime due to such contamination. Similarly, maintenance of such retical inspection tools can be costly when repairs are necessary due to the particle and chemical contamination. Accordingly, it is desirable to allow the EUV light to propagate from the Source Area to the Projection Optics Area, while allowing as little of the contamination to propagate along the same path.
Particle control in light-based reticle inspection is done with flowing air, preventing flow or diffusion of the particles in a known direction, but this method is not broadly possible in a vacuum system. In vacuum systems (such as in electron beam inspectors), particle control is done with slight amounts of positive pressure or particle reduction methods designed to reduce the number of particles in general. In some EUV systems, protection is achieved using high-velocity gas jets moving across the optical path.
One such system and method for protecting an EUV light source plasma production chamber optical element from contamination generated by plasma formation is disclosed in U.S. Pat. No. 7,365,351 (Bowering et al.). This reference discloses a shield system having at least one hollow tube positioned in between the plasma generation site and the optical element. The tube disclosed is arranged to capture debris. A gas system is operatively arranged to release gas into the tube and flow the gas toward the EUV light source.
Japanese Patent Application No. 2005-197456 discloses a technique for controlling ion debris using a magnetic field. Positively-charged ion debris can be collected and/or directed away from the surrounding optical elements by the magnetic field.
U.S. Pat. No. 8,158,959 (Asayama et al.) discloses a device for controlling a flow of ions generated with EUV light including an ion collector which collects ions through an aperture arranged at a side of a chamber, an interrupting mechanism arranged inside the ion collector device, and an ion collision surface which tilts with respect to a direction of movement of the ion.
However, the protection factors created are too low, or they work for particles but not for chemical protection, or they require large amounts of gas flow (and, consequently, large and expensive pumping systems). There is no system that can be arranged to supply a particular amount of gas to the plasma generation site and simultaneously supply another amount of gas to the optical element (larger or smaller) based on the individual requirements of the system.